1. Field of the Invention
The present invention relates to a gravure printing ink as well as a method of multi-color gravure printing on flexible substrates with liquid radiation curable inks, where individual colors are transferred via direct contact between engraved gravure cylinder and the substrate. Each color is dried at an elevated temperature, such that the sequentially printed ink layers transfer through the press without any distortion of the printed image during multiple web turns around turn bars; and finally curing the printed substrate by exposing it to an electron beam (EB) unit.
2. Description of the Related Art
There are three main printing methods used in commercial, high volume printing today, namely offset lithography, gravure and flexography. From a quality standpoint both offset and gravure printing methods are superb and comparable, followed by flexography. In terms of printing mechanism, with both offset and flexography an inked image is transferred from a printing plate containing the image to be printed, to a substrate via an intermediate rubber blanket. In gravure printing, however, the image is transferred directly from an anilox cylinder containing the image to be printed, to the substrate. Inks used for the three printing methods are also very different, primarily in terms of viscosity and pigment concentrations. At room temperature, offset inks have a paste consistency (˜10,000 Poise). Flexo inks have consistency of honey (˜500 centipoises); while gravure inks have very low viscosity (˜50 centipoises).
One of the problems with printing ink formulations is the presence of volatile organic compounds (VOC) in them. This makes the inks subjected to regulation by the EPA and creates a source of additional expenses for the printers. Over the last two decades a certain percentage of the inks used in offset and flexo printing markets have been formulated with VOC free energy curable, ultraviolet (UV) and electron beam (EB) materials. These inks require exposure only to an UV light or an EB unit for instant curing, as opposed to requiring radiant heat sources for drying. The crosslinking associated with the curing process imparts the well known benefits of energy curing, e.g., product and abrasion resistance; high gloss; instant processibility, etc. In comparison, gravure inks are made with either solvents (acetates, alcohols etc.) or water in order to maintain a very low viscosity. However, because of extremely high surface tension (78 dynes) of water, and related substrate wetting and printability problems, water based gravure inks typically contain up to 30% of solvents, in order to lower the surface tension of the inks. So far gravure inks remain a source of VOC emission and require appropriate environmental control for all printers using this printing method. This invention provides a gravure printing method without having any VOC emission restriction.
Typically in gravure printing processes, different colors are printed from one printing station to the next. At each printing station, a gravure cylinder containing the image to be printed is transferred onto a substrate through direct contact. A printed web travels through a drier (typically forced hot air drier) where the VOC component of the ink is evaporated and captured for subsequent processing. The printed web then travels to the next printing station and the process is repeated following the desired color sequence. Once all the colors are printed, the web is passed through a long drier for complete ink drying. For absorbent substrates, e.g., paper, the drying must be good enough to minimize the presence of residual solvents. Whereas for non-absorbent substrates, e.g., film, the drying must be good enough for the printed image not to smear and/or back transfer. For food packaging applications such controls are essential. The evaporated solvents during the drying process are either captured through chilling equipment and recycled or are burned off through adequate thermal oxidizing equipment before exhausting, following appropriate EPA guidelines.
Gravure printing itself is a century old art and is widely used by printers throughout the world. However, use of energy curable inks with gravure viscosity, containing no VOC and the printed image cured instantly upon exposure to an EB unit at the end of the print line is new. Printing with energy curable liquid inks are known. U.S. Pat. No. 6,772,683 describes wet trapping of energy curable liquid inks on a central impression (CI) flexographic press. At each printing station liquid ink is transferred from an anilox roller to a printing plate containing the image to be printed; which in turn transfers the image onto a flexible substrate, compared to a direct transfer from an anilox roller to the substrate in case of gravure printing. In between two subsequent printing stations the water contained in the ink formulations naturally evaporates just enough for the printed image layer to raise its viscosity slightly higher than the next oncoming ink layer, such that the next ink layer can wet trap over the previous one. The ink formulations containing water are specifically designed where the water forms a single phase or homogeneous solution and not a multi-phase dispersion. Therefore only water soluble components can be used in the ink compositions which would form a homogeneous, single phase system and not a heterogeneous or multiphase system. Such compositions and processes are described in U.S. patent application 2008/0233306 A1, and U.S. Pat. Nos. 6,803,112 and 6,011,078. The wet trapped ink layers are exposed to an EB source at the end of the printing process to completely cure the printed matter. The limitation of the process, however, is that wet trapping is only possible where the multiple ink layers do not face a hard surface, e.g., a turn bar on the press, until they are completely cured with an EB. Any such contact prior to EB curing would simply rub off or smear the image onto such a hard rollers, as the ink layers, although capable of wet trapping, are not dry enough to withstand any abrasive surface. As a result, the printing process is restricted to a CI-flexographic printing process only where there are no turn bars on the press, until after the EB unit.
U.S. patent publication 2006/0165997 describes a method for making lottery tickets with improved surface protection and security features, where multiple layers of full coatings are applied onto each other; drying each coating layer with hot air or IR in between each coating station; and finally exposing to an EB unit at the end of the press to completely cure all the coating layers. There is no image on image application.
The present invention describes a gravure printing method which essentially eliminates the volatile solvents present in the gravure inks, and hence will not require any of the VOC controlling equipment required by the EPA. The invention discloses a gravure printing method with EB curable liquid inks having the required viscosity, which are dried, e.g. by a forced hot air drier at each printing station and at the end, the completely printed web is exposed to an EB unit for complete curing of the printed image. An EB curable overprint varnish or adhesive may be applied over the dried printed image and the composite structure cured together by exposure to the EB unit. The method allows gravure printing with its original quality; without any VOC or residual solvent issues; and with all the associated improvements offered by energy curable inks.